1. Field of the Invention
The embodiments of the present invention relate to a poly-silicon-germanium (poly-SiGe) gate stack for semiconductor device and a method for forming the same.
2. Description of the Related Art
Transistor drive currents and hence switching speeds for CMOS (complementary metal oxide semiconductor) devices increase with increasing inversion capacitance. One of the factors that limit inversion capacitance is charge carrier depletion in conventional poly-Si gate electrodes during inversion. Alternative gate electrode materials for CMOS devices used in high performance logic circuits are needed to circumvent polysilicon (or poly-Si) depletion.
Using metal gates, whose charge carrier concentrations exceed the carrier concentration of poly-Si by at least two orders of magnitude, virtually eliminates the poly-Si depletion effect. However, the integration of metal gates into a conventional CMOS flow is complicated due to process integration difficulties. Primarily, metal gates lack the thermal and chemical stability that is necessary to survive subsequent high temperature anneals. Deposition and patterning of metal gates are also presently immature.
An alternative approach is to use poly-SiGe (poly-silicon-germanium) gates instead of conventional poly-Si gates. Germanium (Ge) incorporation into poly-Si enhances doping to increase the amount of implanted dopants that can be activated. This is particularly true of dopants such as boron. The higher dopant activation results in a higher charge carrier concentration and a concomitant reduction in gate electrode depletion during inversion. In addition, SiGe is a mid-gap semiconductor material so that threshold voltages for NMOS and PMOS devices are almost equal in magnitude with opposite polarity. Complementary threshold voltages are necessary for proper CMOS circuit operation. A further advantage of poly-SiGe is that the material is chemically similar to poly-Si. This calls for minimal adjustment to the process integration scheme to accommodate poly-SiGe. Poly-SiGe is thermally stable and can survive the thermal cycling that devices are subjected to during the fabrication sequence.